Carbon nanotubes have excellent conductivity and mechanical properties or the like, and thus there have been studied and attempted to use the carbon nanotubes as conductive materials, or reinforcement materials of various resin compositions, or the like, in various fields and applications such as a conductive paste composition, a conductive ink composition, a composition for forming a heat dissipation substrate, an electrically conductive composite, a composite for EMI shielding, or a conductive material for a battery.
These carbon nanotubes are mainly synthesized by a method such as CVD. The carbon nanotubes thus synthesized have generally a shape of carbon nanotube bundles in which long carbon nanotubes having a length of several tens of an are entangled like a skein of yarn. However, in order to take advantage of characteristics such as excellent conductivity and mechanical properties of carbon nanotubes, there is a need to uniformly obtain carbon nanotubes having a small length and size by pulverizing or disentangling these carbon nanotube bundles.
Previously, there have been known a number of methods for pulverizing or disentangling carbon nanotube bundles. Among them, wet or dry physical methods have mainly been used. Of course, methods for chemically pulverizing or disentangling carbon nanotube bundles have also been considered, but in this case, many defects may occur on the finally formed carbon nanotubes, which may lead to degradation in the properties of the carbon nanotubes.
As the above-described dry physical pulverization or disentanglement methods, methods such as ball milling or jet milling are mainly applied. And as the wet physical pulverization or disentanglement methods, the methods for pulverizing or disentangling carbon nanotube bundles by uniformly dispersing carbon nanotube bundles in a liquid medium together with a dispersant and performing solution-based ball milling, or by a method using ultrasonic irradiation are mainly applied.
However, with the dry physical pulverization or disentanglement method previously known, the size of carbon nanotube bundles can be reduced to some extent, but actually there is a limit to untangle and disentangle the tangled carbon nanotube bundles. Therefore, a large number of particles in the form of tangled bundles are present even in the finally produced carbon nanotubes. This make it difficult to effectively use the carbon nanotubes while taking advantage of characteristics such as excellent conductivity of the carbon nanotubes.
Furthermore, as the solution-based wet ball milling method has also similar problems, the tangled bundles cannot be effectively disentangled and this makes it difficult to effectively use the finally produced carbon nanotubes. In addition, in the case of the ultrasonic irradiation method among the wet pulverization or disentanglement methods, the effect of pulverizing carbon nanotube bundles is not sufficient and thus, there is a disadvantage that, in order to pulverize or disentangle these bundles, there is need to proceed the step for a long period of time and it is difficult to apply to a mass production of carbon nanotubes.
Due to the problems of those prior art methods, there is a continued need for the method for preparing carbon nanotubes which are capable of effectively disentangling or pulverizing the carbon nanotube bundles entangled like a skein of yarn and easily preparing carbon nanotubes having a uniform and fine size.